This invention relates in general to the control of vehicle regenerative braking and deceleration, using the turn signal activation status as input.
As a normal course of driving a vehicle on public roadways, a driver directs the vehicle to a destination through a combination of straight travel and turns. Left and right turns of all kinds are just a normal, ongoing part of driving. As a vehicle on a roadway is approaching an intended turn and before the turn is executed, the driver will usually slow the vehicle from a normal driving speed to a lower speed that is appropriate to safely execute the turn. The execution of a turn therefore requires the driver to anticipate, plan, and act well before the actual turn is made. These steps usually consist of the following: Remove the foot from the throttle, and then apply the foot brake if necessary to achieve a lower speed, release the foot brake and then turn the steering wheel to execute the turn, and then straighten out the steering wheel and re-apply the throttle to complete the turn event. In addition, the driver is required by law to turn on the directional turn signal at a distance before the turn is executed. The main function of the turn signal is to communicate a near term future intended vehicle path to surrounding drivers, thus allowing them to react accordingly to the impending turn. This serves to prevent ambiguity and confusion between drivers, prevents the need for abrupt action countermeasures and reduces the risk of a crash. Use of the turn signal is required by law to be activated a minimum of 100 feet before a turn in most jurisdictions and good practices would dictate a turn signal to be activated approximately 300 to 600 feet before the turn to be an effective communication means to surrounding drivers.
A driver of a vehicle applying the foot brake before a turn will slow the vehicle by converting a portion of the kinetic energy of a vehicle in motion into heat at the designed-in brake friction surfaces. Not only is this heat energy dissipated through the components of the vehicle and eventually to the atmosphere, but brake frictional surfaces experience wear each time the brake is applied and with normal ongoing use. The brake system components eventually will need to be replaced as they wear out and the more frequently the brakes are used and the greater the pressure applied, the sooner the components will wear out. It should be noted there are other speed-reducing forces that will inherently slow down the vehicle that are always present when the vehicle is moving, including aerodynamic drag, rolling resistance of the tires, and internal friction of moving vehicle components and thus, these forces are not relevant to the discussion in this specification due to their ubiquitous and unavoidable presence.
When the driver wants to slow the vehicle down, in particular when approaching a turn, there are methods other than the use of the foot brake to achieve this. These are active means of vehicle deceleration that require a conscious engagement from the driver and will then introduce a reverse torque load at the wheels, thus reducing the speed of the vehicle. Using means other than the foot brake to slow the vehicle reduces the wear on the brakes and extends the life of those related components. One method is for the driver shift the transmission to a lower selected gear which increases engine speed and the higher revving engine will slow the vehicle. In a vehicle equipped with a continuously variable transmission, the ratio can be changed to a higher effective ratio which increases engine speed and thus the higher revving engine will slow the vehicle. In a vehicle with a diesel engine that is so equipped, an exhaust brake may be engaged which introduces increased engine back pressure and thus will slow the vehicle. Also in a diesel vehicle, a compression release engine brake can be engaged resulting in a slowing of the vehicle. In a hybrid electric vehicle (HEV) or an electric vehicle (EV), regenerative braking can be engaged which will result in a slowing of the vehicle. The magnitude of the regenerative braking can be controlled by vehicle design and computer software algorithm combined with the normal, on-the-road driver inputs. Regenerative braking has the added benefit of being able to store a portion of the vehicle's forward motion kinetic energy such that the stored energy can be used at a later time to propel the vehicle.
In the case of the HEV or EV, the manufacturer's design of the vehicle with respect to active means of deceleration, e.g. regenerative braking, presents a dilemma in terms of a design compromise. The recapture and storage of kinetic energy is beneficial, but only when the driver wants to slow the vehicle at a higher rate of deceleration. There are driving situations where simply coasting the vehicle to a lower speed is more appropriate than using regenerative braking, particularly for improving overall fuel economy. Coasting, which is sometimes referred to as gliding, is a method of simply using the vehicle's forward motion kinetic energy to continue to propel the vehicle with minimal drive train negative torque input until a lower speed desired by the driver is achieved. This method is highly efficient when compared to regenerative braking for ultimate kinetic energy utilization, as coasting is near 100% efficient, whereas the regenerative braking-storage-restored kinetic energy cycle is only about 30 to 60% efficient. In comparison, using friction foot brakes would be considered about 0% efficient since energy is converted to heat and dissipated to the atmosphere.
Therefore, the dilemma for the manufacturer designing a HEV or EV is how to control coasting and how to control regenerative braking with a proper balance with respect to driver's controls and timely needs in particular driving situations. A vehicle manufacturer's design dictates the amount of regenerative braking and the amount of coasting that a HEV or EV will exert under various driving circumstances. With fixed design parameters established with computer control algorithms, inputs from the driver combined with vehicle dynamics then determine the regenerative braking profile. A manufacturer must consider drivability, driver skill levels, driver interaction to vehicle function and operation, vehicle internal complexity, cost, overall fuel mileage, safety, vehicle durability and reliability among other considerations when designing these controls. At one extreme, a vehicle manufacturer could design the vehicle such that the regenerative braking is at or near a maximum when a driver releases the throttle to zero percent, that is, removes the foot from the throttle. With this type of throttle lift, the entire vehicle including the driver and any passengers will likely experience a very high rate of deceleration. In this design compromise scenario, maximum regenerative braking is occurring and minimal coasting is occurring. Thus, in a HEV, battery maximum capacity may be reached very frequently, thereby reducing the opportunity to recapture forward motion kinetic energy of the moving vehicle. This also may compromise the drivability of the vehicle with frequent, sudden decelerations when the driver's foot is lifted off the throttle. This design is prone to result in a jerky ride with aggressive fore and aft movements. Furthermore, the opportunity to translate forward motion energy into forward motion with coasting is not present. Also, throttle-position-induced rapid deceleration can create the situation whereby a rear end crash from a following driver is an ever-present risk, or the vehicle may experience instability. At least one commercially sold vehicle, the Tesla Roadster EV, has designed such a vehicle with maximum regenerative braking and has equipped the vehicle so that the brake lights are illuminated while under the throttle-position-induced rapid deceleration conditions described.
Other manufacturers' design with a combination of regenerative braking and coasting and consists of two states of operation that can be induced by the driver's inputs: In the first state, when a driver lifts their foot off the throttle, a minimal amount of regenerative braking occurs. But when the brake pedal is depressed in the first approximately 30% threshold of the brake pedal travel, then a progressive level of regenerative braking is introduced. In this design, brake pedal travel past the threshold engages the conventional friction brake system of the vehicle in conjunction with the regenerative braking. There exists a second state of this design when the driver's foot is lifted from the throttle, then momentarily taps the throttle pedal to then engage a coasting mode. In this state, minimal, if not zero magnitude induced drivetrain load or regenerative braking exists, thereby making maximum use of forward kinetic energy to propel the vehicle most efficiently. While the second state maximizes overall efficiency by utilizing the coasting feature, a majority of drivers will not make the conscious effort to engage this second state, thereby missing the opportunity to optimize on a large population of these vehicles. The aforementioned design approach is utilized on the current model of the Toyota Prius HEV.
Still another compromise design method of present HEV and EV manufacturers is to have a driver selectable switch whereby fixed mode profiles with predetermined levels of coast, regenerative braking and friction braking. These selectable operational modes are available to the driver such that greater or lesser magnitudes of regenerative braking levels are induced with normal driving and specifically lifting the throttle, depending on the mode selected. The driver must make a choice of one or more modes and this selectable approach does not allow the vehicle to be automatically adaptable to the situation. This approach also misses the opportunity to optimize on a large population of vehicles. The aforementioned design approach is utilized on the current model of the Nissan Leaf EV.
In the case of a conventional engine/transmission vehicle with no internal means to convert and store vehicle forward kinetic energy, there is still an advantage to being able to slow the vehicle through downshifting or other means when approaching a turn. This would create the advantage of less wear on the friction brake system components. Less wear means less heat and thus a longer time between component wear-out and vehicle service.
While all of these methods of achieving vehicle deceleration and regenerative braking are designed by the manufacturer and are executed by a driver's actions while normally driving the vehicle, all vehicle systems are reactionary to realtime vehicle dynamics resulting predominately from the driver's throttle, brake, and steering inputs. It remains an overriding and ongoing design challenge to extract greater vehicle efficiency and is requiring that both manufacturers and drivers seek incremental ways to improve both fuel mileage and vehicle range and methods that adapt automatically to optimize efficiency are the most desirable. Thus, the pursuit of further optimizing regenerative braking systems is a worthwhile effort, with benefits to be gained for all interested parties. Fuel mileage is taking on a more and more important role and the United States Environmental Protection Agency corporate average fuel economy (CAFE) benchmarks continue to climb so vehicle manufacturers must therefore pursue cost effective means of improving fuel economy in all conceivable areas. Energy efficiency and energy management, specifically when a vehicle is engaged in a turning event, is where the prior art is non-existent and therefore has potential for improvement. None of the approaches outlined, nor any other prior art recognizes or utilizes the synergy of a driver's activated turn signal as an early indicator to the vehicle control system that the driver is intending to slow down, then take advantage of this information to optimize energy resource management regarding the turn event.
What is needed therefore is a cost effective system whereby the vehicle could accurately and automatically anticipate when a driver is about to execute a turn and thus optimize regenerative braking and/or energy dissipative braking well before the turn is executed. These functions in various combinations could result in greater energy recapture of forward motion kinetic energy and thus increase fuel economy, increase a manufacturer's CAFE numbers, increase vehicle range, reduce operating costs, improved vehicle drivability, promote the proper use of the turn signal and reduce the wear on the friction braking system.